The present invention relates to a process for eliminating water vapor from gaseous mixtures by selectively separating water vapor from moisture-laden air, hydrocarbon gases such as natural gas or other industrial process gases.
Methods for removing water vapor from air, separating water from hydrocarbon gases in natural gas or removing water vapor in the manufacturing or purification processes of various types of gases include methods in which water is condensed and removed by compressing the gas that is to be treated and methods in which the moisture content is condensed and removed by cooling. However, such methods require a large driving force or thermal energy for compression or cooling.
The use of moisture-absorbing agents such as CaCl.sub.2, NaCl and LiBr is also known. However, the use of such a method requires a moisture-absorbing agent. Because continuous moisture absorption is difficult, it is difficult to apply such a method to an industrial production line which requires continuous operation to be efficient.
A method which removes water vapor by means of a membrane using the difference in the partial pressure of the water vapor has been proposed. The use of such a membrane method makes continuous operation possible. Furthermore, water vapor can be separated using only a relatively small amount of energy. Examples of such processes have been disclosed in Japanese Laid-Open patent application (Kokai) No. 54-114481, in which PVA containing an aqueous solution of lithium chloride is used; Japanese Laid-Open patent application (Kokai) No. 54-152679, in which a cellulose-type membrane is used; Japanese Laid-Open patent application (Kokai) No. 60-261503, in which an organosilane plasma-polymerized membrane is used; Japanese Laid-Open patent application (Kokai) No. 62-42722, wherein a polyamino acid urethane is used; Japanese Laid-Open patent application (Kokai) No. 62-42723, wherein an aromatic polyimide is used; U.S. Pat. No. 3,735,558; Japanese Laid-Open patent application (Kokai) No. 61-187918; Japanese Laid-Open patent application (Kokai) No. 61-229830; Japanese Laid-Open patent application (Kokai) No. 62-7417; and Japanese Laid-Open patent application (Kokai) No. 62-30338, wherein hollow fluororesin-type membranes are used.
In the case of conventional membrane processes, sufficient water vapor selectivity and permeation rate are not obtained. Accordingly, such processes have not yet been widely employed on an industrial level.
Even in the case of processes disclosed very recently, the water vapor permeation rate is relatively low so that a large-scale apparatus is unavoidably necessary in cases where a large-scale industrial treatment is required.
Furthermore, in the case of conventional processes such as those mentioned above, the selective separation properties are not always desirable, either. The separation ratios obtained are generally low, and, in this respect, the processes are disadvantageous in terms of apparatus and operation.
Membranes which are selectively permeable to water vapor and also impermeable to other gases and liquid water are also known, as, for example, in U.S. Pat. No. 4,194,041.
By definition herein, the flow rate of water vapor through a separation membrane is designated Q.sub.H.sbsb.2.sub.O, the flow rate of other gases that are to be separated from the water vapor is designated Q.sub.G and the ratio of water vapor flow rate to the flow rate of the gas that is to be separated from the water vapor, i.e., Q.sub.H.sbsb.2.sub.O /Q.sub.G, is designated as X.
Even in theoretical terms, it has been difficult to obtain a membrane material with a high Q.sub.H.sbsb.2.sub.O and a high X ratio in the range of industrially feasible membrane thicknesses.